1. Field of the Invention
This invention relates generally to oxygen inhibition in polymerization.
2. Background
Free radical polymerization is by far the most widely used chain polymerization technique for industrial applications. These industrial applications include, for example, thin films, coatings, coating and paint industries, adhesives, optics, dental filling, sealing compound, and stereo-lithography. These reactions offer many advantages over other polymerizations, including 1) high reaction rates, 2) insensitivity to impurities (compared to anionic and cationic polymerizations), and 3) a wide selection of commercially available monomers and oligomers. However, the vulnerability of free radical polymerizations to inhibition by molecular oxygen remains a significant problem with the technique.
Oxygen inhibition is perhaps the most important unsolved problem in free radical polymerization. Molecular oxygen, with its extraordinary biradical structure and high reactivity towards electron rich groups, participates in numerous chemical and biochemical processes which in some degree determine the ultimate outcome of these reactions. The possible interactions between oxygen and the polymerization system include two mechanisms:    1) physical quenching of the triplet state of the initiator or sensitizer and    2) scavenging of the free radicals/active radical centers (primary or the propagating chain) to produce unreactive peroxide radicals.Both of these mechanisms of oxygen inhibition will significantly reduce the polymerization rate until the oxygen in the system is consumed. Oxygen inhibition 1) reduces the polymerization rate, 2) may reduce the primary polymer chain length, and 3) limits the ultimate attainable conversion in polymerization systems. The presence of oxygen in free radical polymerization systems is known to be the primary cause of an inhibition period and will ultimately affect the attainable properties of the polymer. In an open system where the oxygen will diffuse into the sample incessantly, an incomplete surface cure of the polymer will generally be observed.
Discovering an efficient way to eliminate oxygen inhibition has been a long-standing goal of polymerization scientists and engineers. As a result, a number of approaches have been tried to mitigate the effect of oxygen in free radical polymerizations. Because no satisfactory method has been reported, polymerization often must be carried out under an inert nitrogen atmosphere using expensive inerting equipment.
Many of the current approaches involve the creation of an oxygen barrier (for example, paraffin wax) to prevent the diffusion of oxygen into the system. These barrier approaches are not widely applicable since in many cases it is impractical to add a barrier layer.
The most common method to counter the effect of oxygen is simply to add enough initiator to create enough active centers to both react with the oxygen and polymerize the monomer. This method is not very satisfactory for many cases since the presence of the oxygen still leads to an inhibition period and reduces the length of the polymer chains (often leading to a tacky surface even after cure).
Other current methods for mitigating the effect of oxygen on free radical polymerizations are based on oxygen scavengers (such as tertiary amines). These methods are not satisfactory for many reactions and are fundamentally different from the invention described herein. For example, one way in which prior methods differ is use of a tertiary amine radical capable of forming a hydroperoxide is also capable of initiating the polymerization, therefore, it is not possible to decouple the oxygen depletion and the polymerization using this oxygen scavenger technique (therefore, the deleterious effect on the molecular weight will still be present using oxygen scavengers). In addition, it is often undesirable to add an amine to the reaction system, since the presence of the amines could lead to a residual odor in polymer product, may be toxic, and may lower the shelf life of the formulation.
Two papers published by Christian Decker (C. Decker, Makromol. Chem. 180, 2027, 1979 and C. Decker, J. Faure, M. Fizet, and L. Rychla, Photographic Science and Engineering, 23, 137, 1979) are directed toward addressing the oxygen inhibition problem. In both of these papers, Decker reports using the dye methylene blue (excited using a flash lamp with a 500–800 nm filter) to photochemically transfer ground state oxygen into singlet oxygen and then reacting the singlet oxygen with the compound diphenylisobenzofuran (DPBF) to consume the oxygen. The resulting oxidation product, orthodibenzoylbenzene, may further act as an efficient photoinitiator of the polymerization.
While both the Decker method and the current method involve the production of singlet oxygen by reaction with a light-absorbing molecule, followed by the reaction of the singlet oxygen with a second compound, there are many important differences between the system reported by Decker and the current invention. Some of the differences are    1) The singlet oxygen generators used in the current invention are much more versatile than the Decker dye sensitizer, methylene blue (the current singlet oxygen generators are soluble in a wider range of monomers and exhibit unique photochemical properties that are not offered by methylene blue).    2) The singlet oxygen generators used in the current invention do not lead to the production of active centers by themselves or by interaction with common initiators. Methylene blue will photoinitiate polymerization by itself and will interact with common initiators and coinitiators (such as amines) to create active centers.    3) The singlet oxygen generators used in the current invention undergo much less photobleaching than methylene blue and rose bengal during pre-illumination thereby allowing much more efficient production of singlet oxygen.    4) The only compound Decker described as the singlet oxygen acceptor is DPBF (1,3-diphenylisobenzofuran). DPBF does not meet the criteria (described below) as a singlet oxygen trapper because, for one thing, it is not stable in most monomers (especially acrylic monomers)—the C═C double bond in an acrylic monomer is a good dienophile and has a strong possibility to undergo a Diels-Alder reaction with DPBF (studies have shown that the compound decomposes rapidly in 2-hydroxyethylmethacrylate (HEMA) and butylmethacrylate monomers, totally degrading in less than 30 minutes). This drawback significantly limits the application of DPBF in polymerization systems.
The “consumption” of oxygen prior to the initiation step, as described below for the current invention, is clearly a different fundamental approach than other methods of dealing with oxygen inhibition. The current invention allows the initiation and oxygen depletion to be decoupled, which is not possible if using excess initiator to deplete the oxygen, providing flexibility in the design of the polymerization process. The current invention will reduce the cost of free radical polymerization by eliminating the need for nitrogen purging and its associated complicated equipment. The current invention also allows oxygen to be consumed before the polymerization begins, thus, leading to an increase in primary polymer chain length.